The present disclosure relates to a fixing device to fix a toner image on a sheet and an image forming apparatus including the fixing device.
As a configuration of a fixing device to fix a toner on a sheet, the following configuration is well known. The fixing device having such a configuration includes a pressing roller having a core metal and an elastic layer, a rotating member (a fixing roller, a fixing belt or the others) coming into contact with an outer circumferential face of the pressing roller, a heating means to heat the rotating member and a pressing mechanism to press the pressing roller to the rotating member to form a pressing area through which the sheet is conveyed, between the pressing roller and the rotating member. Around an outer circumferential face of the rotating member, a release layer made of fluorocarbon resin is formed in order to promote the releasing of the toner.
In the above fixing device, a technique to release the sheet adhered on the rotating member from the rotating member is known. For example, there is a technique that a wedge-shaped or a plate-shaped separation member is arranged at the downstream side of the pressing area, and a tip end of the separation member buts against the outer circumferential face of the rotating member to separate the sheet from the rotating member. However, on butting the separation member against the rotating member, the release layer is scraped by the separation member and abraded. If the abrasion of the release layer proceeds, the melt toner may adhere to the rotating member or the adhered toner may be transferred on the next sheet to cause a toner offset.
Then, a technique for protecting the surface of the rotating member is discussed. For example, a configuration is proposed, in which a contact pressure of a separation claw (the separation member) to the fixing roller is increased at a timing when an end of a recording medium (the sheet) is passed through a contact position of the separation claw to the fixing roller and the contact pressure of the separation claw to the fixing roller is decreased at a timing after the end of the recording medium is passed through the contact position of the separation claw to the fixing roller.
By the way, as material of the elastic layer of the pressing roller, silicon rubber is conventionally employed. However, a rigidity of the silicon rubber is decreased with time. Particularly, these days, to meet a request for energy saving, foamed silicon rubber having a low heat capacity is frequently employed. Because the foamed silicon rubber has a volume ratio of the rubber to the whole volume lower than that of a solid rubber containing no pore, the rigidity is easily decreased. If the rigidity of the elastic layer is decreased, a deformation amount of the elastic layer is increased to lengthen the pressing area in the sheet conveyance direction and then to increase a heat volume transferred to the sheet. Then, an agglomerating force of the toner is decreased so that the sheet is easily separated from the rotating member.
However, in the above configuration, because a load applied on the rotating member by the separation member is not varied even if the rigidity of the elastic layer of the pressing roller is decreased, the abrasion of the rotating member proceeds by an excessive load.